1. Field of the Invention
The present invention relates to a method and an apparatus to perform sampling in chromatographic systems with very small amounts of liquid sample, said method and said equipment being particularly but not exclusively applicable to high resolution gas chromatographic systems, with capillary or micropacked columns, or to thin-layer chromatographic systems. Use of this method and apparatus makes it possible to perform controllable and reproducible sampling of very small amounts of sample, with values unattainable through the techniques usually employed for liquid sampling in chromatographic systems and particularly using micro-syringes or pipettes.
2. Description of the Prior Art
The microsyringes used in chromatography are generally of the type with calibrated body (capable of sampling amounts ranging from 0.2 to 10 microliters) or of the type with calibrated needle, where the piston penetrates into the needle. The latter microsyringes are capable of handling smaller quantities of samples, in a reliable and reproducible way, but only within certain limits, in particular with lower limits of about 200-300 nanoliters. Below this limit, the high surface tension of the liquid and the relatively reduced speed of the piston movement do not allow the drop, which has formed at the needle end, to fall from it, considering the reduced diameter of the outlet nozzle of the needle. Precision is moreover negatively affected by poor sealing between piston and calibrated needle.
Another known system is the sampling system commonly used in the laboratory and named "pipette system", in which a calibrated tubing is filled with a liquid to be transferred, by sucking it into the tubing, the liquid amount placed in the tubing is controlled and the same is retained in the tubing by closing same at the end where aspiration has been carried out. Then, an injection of the liquid into a receiving container is made by opening said end. This sampling method or transfer method of determined amounts of liquid is well known and has been used in gas chromatography too, but however only for quantities usually measurable in a rather rough way, even if the literature reports a lower limit of 25-50 nanoliters (see R. Kaiser-Gas Phase Chromatography--Vol. 1 pp. 90-95--Butterworths 1963--London).
Therefore it can be considered that, of course according to the nature of the liquid substance to be sampled, a lower limit exists, generally between 50 and 200 nanoliters, below which it is not possible to go in reliable and reproducible sampling using microsyringes or micropipettes.
Furthermore, in the case of the pipettes, and also often in the case of the microsyringes, when the gaseous fluid between the piston and the sample liquid is not completely eliminated, considering that a quantitative determination is performed during feeding, it may occur that the gas pushing the liquid out of the pipette, enters the capillary column, which sometimes involves problems connected with the choice and the use of said gas, such that it does not affect analyses.
The above mentioned quantitative limitations, however, are such that the operator is often forced to perform accessory operations imposed by the relatively high quantities of sample that has to be introduced into the chromatographic system and particularly into the capillary column. In fact, especially in the case of direct injection without vaporization (oncolumn), particularly considered herein, the sample must be diluted in a dilution ratio which is often very high (of the order of 1:10000 or more), with an operation which may involve difficulties in the exact analytical determination of the sample and in that it can introduce discriminations or variations in the sample original conditions.
In other cases, a co-called "splitting" operation is necessary, that means the elimination of a high percentage of the quantity fed to the injector, before its introduction into the column, which operation may involve even higher risks of discriminations.